This invention relates to the biological treatment of hazardous waste.
Volatile organic compounds (VOCs) of various kinds are widely used in many industrial processes. Typical VOCs include acetone, toluene, xylene, and methyl ethyl ketone (MEK). Disposal of VOCs or substances containing VOCs represents a significant problem for many industries.
Operation of a paint shop generates a large quantity of solvent-laden rags. In many cases, the solvents are VOCs, and the solvent absorbed in the rags is allowed to evaporate into the atmosphere. In some jurisdictions, paint shops operate under permits that limit the amount of their solvent emissions, and the solvent that evaporates from the rags counts toward that limit. Even if this were not the case, the solvent evaporation would be undesirable as contributing to atmospheric pollution.
A paint can containing unused paint is considered hazardous waste if the residue in the can is liquid. Often the residue is liquid due to the presence of VOCs. If the residue is solid, in most cases the can is no longer considered to be hazardous waste, and it is permissible to dispose of it in a landfill.
A still bottom (the residue remaining in a distillation vessel after useful fractions have been driven off) may contain VOCs. If the still bottom is solid, in most cases it can be disposed of in a landfill, but if it contains VOCs the still bottom must be treated as hazardous waste.
A known technique for solving the problem of paint cans and still bottoms is to allow any volatilizable compounds present in the residue to evaporate at ambient temperature into an effectively unconfined space. Since the VOCs evaporate at a fairly slow rate, and the space into which the VOCs evaporate is effectively unconfined, the concentration of VOCs in the atmosphere in the vicinity of the residue does not normally become bothersome, but this practice nevertheless contributes to atmospheric pollution.
Disposal of used aerosol cans also presents a problem of atmospheric pollution. Generally, the cans contain VOCs as propellant. If a can is discarded, there is a significant possibility that the can will eventually lose its integrity and the VOCs present in the can will escape into the atmosphere.
Compost comprises dead organic material and various microorganisms. The organic material may be a mixture of various types of plant debris, such as leaves, grass, and wood. A plant compost is a complex mixture of solids, some of which have decayed fully while others are in various stages of decay. The organic material provides attachment sites for the microorganisms that are present in the compost and also serves as a source of nutrients, particularly micronutrients such as nitrogen, phosphorus, potassium, and minerals. Some of the microorganisms that are present in compost can use certain VOCs as nutrients and convert them, in the presence of oxygen, into carbon dioxide and water.
A compost may be characterized by its maturity. A fresh, or immature, compost, for example containing fresh grass clippings, newly fallen leaves and new wood chips, contains a relatively small population of microorganisms. The microorganisms extract nutrients from the compost, and this causes the compost to decay. As the population and variety of microorganisms increases, both through addition of air-borne microorganisms and through multiplication of microorganisms that are already present, the decay process accelerates and the compost matures. When the compost is fairly young, the level of biological activity can be such that the compost spontaneously heats up to a significant extent and may destroy the microorganisms. When the compost is fully decayed, the microorganism population dies out and the compost can be used as a mulch, for example.
U.S. Pat. No. 4,544,381 discloses a method for treatment of oxygen-containing gases that are contaminated with biodegradable substances. The treatment is accomplished by bringing the gases into contact with an aqueous suspension of suitable microorganisms.
U.S. Pat. No. 4,662,900 and U.S. Pat. No. 4,806,148 each disclose a method for removal of malodorous and/or toxic components from waste gases by biological treatment. The gas stream is passed through a filter bed that contains an organic biologically active material.
U.S. Pat. No. 5,089,036 discloses an air scrubber that employs microorganisms for digesting organic pollutants produced by experiments conducted in a glove box. The glove box and the air scrubber are in a closed circuit.
U.S. Pat. No. 5,246,584 discloses a method for destroying organic contaminants in an aqueous liquid, particularly ground water. The organic contaminants are volatilized and the vapors are condensed and then processed in a bioreactor, in which microorganisms convert the organic contaminants into carbon dioxide, water, and hydrochloric acid.